a) Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and more particularly to a semiconductor manufacturing method having a step of growing group III-V compound semiconductor layers by organo-metallic vapor phase epitaxy (MOVPE).
Demand for visible light semiconductor lasers is increasing for the application to light sources of optical disk read/write, point of sales (POS) systems, printers, and the like. AlGaInP based semiconductor lasers using GaAs substrates are widely used as such visible light semiconductor lasers.
b) Description of the Related Art
An AlGaInP based visible light semiconductor laser is formed on a GaAs substrate and has a GaInP or AlGaInP active layer sandwiched between AlGaInP clad layers.
FIG. 5A shows an example of the structure of an AlGaInP/(Al)GaInP visible light semiconductor laser according to the conventional technique. On an n.sup.+ -type GaAs substrate 61, an n-type GaAs buffer layer 62 is grown, and then an n-type AlGalnP clad layer 63 is grown, to form n-type regions. A group IV element Si, group VI elements Se, S, and the like, may be used as n-type impurities.
On these n-type regions, a non-doped GaInP or AlGaInP active layer 64 is formed on which a p-type AlGaInP clad layer 65 and p-type GaInP intermediate layer 67a are formed in this order. Thereafter, a SiO.sub.2 striped mask is formed on the surface of the device. Mesa etching is performed using the SiO.sub.2 mask as the etching mask to remove the p-type clad layer 65 down to the intermediate depth.
This mesa etching SiO.sub.2 mask is also used as a growth mask to grow an n-type GaAs current block layer 66 to embed with it both the side surfaces of the mesa structure. The GaAs current block layer 66 confines lateral light and hole current of the active layer 64. The surface is then covered with a p-type GaAs contact layer 68a to obtain a finished semiconductor laser structure.
Also formed are an n-side electrode 69 under the n.sup.+ -type GaAs substrate 61, and a p-side electrode 70 above the p-type GaAs contact layer 68a.
In the semiconductor laser having the structure such as shown in FIG. 5A, it is desired to suppress heat generation in the p-type clad layer 65 by reducing the resistivity thereof as much as possible, and to provide effective confinement of injected carriers. For this reason, it is desirable to increase the doping concentration of the p-type clad layer 65 and p-type intermediate layer 67a as much as possible.
FIG. 5B shows another structure of an AlGaInP/GaInP visible light semiconductor laser. A striped mesa structure is formed on the surface of a Zn doped p-type GaAs substrate 111. A Se doped n-type GaAs current block layer 112 is formed covering both the side surfaces of the mesa structure. A gradually declining weakened mesa structure is then formed on the surface of this device. The current block layer 112 forms a potential barrier against holes transported from the p-type substrate 111, to concentrate hole current above the hill top of the mesa structure.
Formed on such a composite mesa structure are a Zn doped p-type GaAs buffer layer 113 and Zn doped AlGaInP clad layer 115. An intermediate layer 114 has an intermediate band gap between those of the buffer layer 113 and clad layer 115.
On these p-type regions, a non-doped GaInP active layer 116 is formed on which n-type layers are deposited and laminated. First, a Se doped n-type AlGaInP clad layer 117 is formed on which a Se-doped GaInP intermediate layer 118 and Se doped n-type GaAs contact layer 119 are formed. The intermediate layer 118 has an intermediate band gap between those of the clad layer 117 and contact layer 119.
This laminated-layer structure is epitaxially grown on the mesa structure and has a curved surface similar to the mesa structure. With such a structure, a light waveguide is formed above the hill top of the mesa structure. Since the light waveguide is self-aligned with the mesa structure, this laser is called a self-aligned bent waveguide laser.
The intermediate layer 114 having the intermediate band gap between those of the buffer layer 118 and clad layer 115 generates potential barriers, each called a spike, at the both sides thereof, because of the heterojunction. In order to prevent the suppression of current to be caused by this potential barrier, it is desirable to uniformly dope p-type impurities at a high concentration into the buffer layer 113, intermediate layer 114, and clad layer 115, or to use the graded structure providing a gradual change of the band structure. This approach is also applicable to the structure shown in FIG. 5A.
Nishikawa et al say that the doping delay, although it depends greatly upon the growth temperature, and upon whether the matrix is InAlP or InGaP, it can be explained by a model simulating an inability of doping until Mg is accumulated to a certain amount on a solid phase surface (refer to Extended Abstracts of the 22nd, 1990 International Conference on Solid State Devices and Materials, Sendai, 1990, pp.509 to 512).
Hatano et al propose to use adduct of trimethyl aluminum and dimethyl magnesium in order to realize Mg doping without a doping delay.
AlGalnP based materials are generally grown using organo-metallic vapor phase epitaxy. As p-type dopants, Zn is generally doped by the use of a source material of dimetyl zinc (DMZn, (CH.sub.3).sub.2 Zn) or diethyl zinc (DEZn, (C.sub.2 H.sub.5).sub.2 Zn).
However, the vapor pressure of Zn is high and the release or removable of Zn from the crystal surface is conspicuous, resulting in a hardship of high concentration doping (5*10.sup.17 cm.sup.-3 or higher). Diffusion of Zn is large in an AlGaInP based crystal. When doped in an AlGaInP based crystal, Zn generates by-products by the reactions with In or P, which may become nuclei of crystal defects.
Several problems discussed above are associated with Zn when used as p-type dopants. From this reason, Mg in place of Zn has been expected as alternative dopants.
Mg allows high concentration doping (in the order off 2*10.sup.18 cm.sup.-3). Mg diffuses less in an AlGaInP based crystal. Mg will not generate by-products upon reacting with AlGaInP based matrix materials.
Mg used as p-type dopants has many advantages discussed above. However, the problem of doping delay is associated with Mg used as p-type dopants for AlGaInP based materials. Mg will not be doped to a desired amount at the initial stage of the crystal growth. In other words, there is a time delay between the start of supplying Mg and the actual doping of Mg into the crystal.
This doping delay phenomenon is relatively small in AlGaInP based material containing Al, but conspicuous in GaInP, GaAs, or the like not containing Al, posing a serious problem. It is known that the doping delay can be made small if the amount of Mg source material is increased. However, in the range of carrier concentrations practically used, the control of the amount of Mg is difficult, leaving the problem unsolved.
The doping delay phenomenon in Mg doping will be described with reference to FIG. 5C which shows the growth of a Mg doped GaAs epitaxial film on a Zn doped GaAs substrate, with the abscissa representing the depth from the surface and the ordinate representing the dopant concentration.
In the GaAs epitaxial film, the Mg concentration is extremely small at the initial stage of the growth and gradually rises as the growth progresses further, although the same amount of Mg is used during the Mg doping process.
At the initial stage, the GaAs epitaxial film becomes p-type because of the out-diffusion of Zn from the Zn doped GaAs substrate. However, the region from the low Zn concentration area to the gradually rising Mg concentration area becomes a high resistance area because of low impurity (Mg) concentration.
Particular problems when using Mg accompanied with the doping delay as p-type dopant will be described with reference to FIG. 5D.
In FIG. 5D, on a p.sup.+ -type GaAs substrate 71, a p-type GaAs buffer layer 72 is grown on which a p-type GaInP intermediate layer 73 and p-type AlGaInP clad layer 74 are grown to form p-type regions. On these p-type regions, a non-doped GaInP layer 75 is grown as an active layer on which an n-type AlGaInP clad layer 76 and an n-type GaAs contact layer 77 are grown to form n-type regions.
Suppose that Mg is doped as p-type dopant into the GaAs buffer layer 72, GaInP intermediate layer 73, and AlGaInP clad layer 74. In the GaAs buffer layer 72 and GaInP intermediate layer 73, a desired p-type may not be obtained because of the difficulty in doping Mg. In some cases, these regions become n-type. Without a desired amount of doped Mg, the finished diode structure will not allow current to easily pass or degrade its I-V characteristics.